1. Field of the Invention
The present invention relates to a computerized tomographic (CT) apparatus utilizing radiation rays such as X-rays, and, more particularly, to a CT apparatus which performs a helical scan to acquire projection data.
2. Description of the Related Art
CT apparatuses of the above mentioned type recently became popular since they can scan a wide examining region of an object to be examined in a short period of time. One example of such a CT apparatus is the computerized tomographic apparatus utilizing a radiation source disclosed in U.S. Pat. No. 4,630,202. The helical scan system moves the object in a given direction by permitting the patient platform of a gantry assembly on which the object is placed to slide in the direction of the body axis of the object, and rotates an X-ray tube around the object to continuously scan it. If it is assumed that the object is fixed, the locus of the movement of the X-ray tube relative to the object forms a helix. The pitch of the helix corresponds to the velocity of the patient platform. In contrast ordinary dynamic scan and multi-scan systems fix the object and the X-ray tube is always rotated within the same slice.
In reconstructing an image from projection data acquired by the helical scan, system projection data for one helical turn may be processed as one set of projection data corresponding to one slice of the image. In this case, however, since the data for a projection angle of 0.degree. differs from that for a projection angle of 360.degree. by a distance corresponding to the pitch of the helix, some inconsistency will naturally occur in image reconstruction. This is likely to cause an artifact, such as a streak on a reconstructed slice image. This artifact becomes more prominent as the distance corresponding to the pitch of the helix is increased to speed up data acquisition.
To prevent such shortcomings, according to the aforementioned United States patent, data from 0.degree. to (360+.beta.).degree. is acquired as one set of projection data, and data from 0.degree. to .alpha..degree. is smoothed using data from 360.degree. to .alpha..degree. in order to eliminate the above inconsistency. This method, however, simply causes the data for 0.degree. and the data for 360.degree. to appear within the same slice, and does not cause data for the entire 360.degree. to come within the same slice (plane). Therefore, the artifact cannot be eliminated completely.